New retroviral vectors were developed for potential use in human gene therapy. Enhancement of expression and/or expansion of tropism for current retroviral vectors by replacing Moloney LTR U3 sequences with those from various other retroviral LTR's was attempted. Retroviral LTRs tested included the standard Moloney MLV, AKV MLV, NZB-Xeno MLV, the Harvey sarcoma virus, the myeloproliferative virus, and the sarcoma leukemia virus. This battery of chimeric LTR vectors will allow the selection of a vector backbone suitable for a variety of gene therapy protocols.